Programmable photolithographic mask system and method

ABSTRACT

The present invention overcomes many of the disadvantages of prior lithographic microfabrication processes while providing further improvements that can significantly enhance the ability to make more complicated semiconductor chips at lower cost. A new type of programmable structure for exposing a wafer allows the lithographic pattern to be changed under electronic control. This provides great flexibility, increasing the throughput and decreasing the cost of chip manufacture and providing numerous other advantages. The programmable structure consists of an array of shutters that can be programmed to either transmit light to the wafer (referred to as its “open” state) or not transmit light to the wafer (referred to as its “closed” state). The programmable structure can comprise or include an array of selective amplifiers. Thus, each selective amplifier is programmed to either amplify light (somewhat analogous to the “open” or “transparent” state of a shutter) or be “non-amplifying” (its “closed” or “opaque” state). In the non-amplifying state, some portion of the incident light is transmitted through the amplifier material. The shutters and selective amplifiers can work in tandem to form a “programmable layer”. A programmable technique is provided for creating a pattern to be imaged onto a wafer that can be implemented as a viable production technique. Thus, the present invention also provides a technique of making integrated circuits. A diffraction limiter can be used to provide certain advantages associated with contact lithography without requiring some of the disadvantages of contact lithography.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation of application Ser. No.10/166,615 filed Jun. 12, 2002, now Patent No. ______; which is acontinuation of application Ser. No. 09/871,971 filed Jun. 4, 2001, nowPatent No. 6,480,261; which is a division of application Ser. No.09/066,979 filed Apr. 28, 1999, now Patent No. 6,291,110. Thisapplication claims the benefit of U.S. Provisional Application No.60/051,121 filed Jun. 27, 1997 entitled “A Shutter For a ProgrammablePhoton Lithography Mask”; U.S. Provisional Application No. 60/058,701filed Sep. 12, 1997 entitled “A Doped Solid-State Lithography Mask”;U.S. Provisional Application No. 60/058,702 filed Sep. 12, 1997 entitled“A Device to Improve Resolution In Lithography Using A ProgrammableMask”; and U.S. Provisional Application No. 60/060,254 filed Sep. 29,1997 entitled “A Selective Amplifier For a Programmable PhotonLithography Mask.” The entire contents of these are hereby incorporatedby reference in this application.

FIELD OF INVENTION

[0002] This invention relates to microfabrication and more particularlyto systems, methods, and techniques for manufacturing integrated circuitchips using photon lithography. Still more particularly, the presentinvention relates to systems, methods, and techniques in connection withprogram able masks for microlithography.

BACKGROUND AND SUMMARY OF THE INVENTION

[0003] Lithography is used to transfer a specific pattern onto asurface. Lithogaphy can be used to transfer a variety of patternsincluding, for example, painting, printing, and the like. More recently,lithograohic techniques have become widespread for use inmicrofabrication—a major example of which is the manufacture ofintegrated circuits such as computer chips.

[0004] In a typical microfabrication operation, lithography is used todefine patterns for miniature electrical circuits. The lithographydefines a pattern specifying the location of metal, insulators, dopedregions, and other features of a circuit printed on a silicon wafer orother substrate. The resulting semiconductor circuit can perform any ofa number of different functions.

[0005] Improvements in lithography have been mainly responsible for theexplosive growth of computers in particular and the semiconductorindustry in general. The major improvements in lithography can, for themost part, be put into two categories: an increase in chip size, and adecrease in the minimum feature size (improvement in resolution). Bothof these improvements allow an increase in the number of transistors ona single chip (and in the speed at which these transistors can operate).For example, the computer circuitry that would have filled an entireroom in 1960's technology can now be placed on a silicon “die” the sizeof your thumbnail. A device the size of a wristwatch can contain morecomputing power than the largest computers of several decades ago.

[0006] One type of lithography that is commonly used in the massproduction of computer chips is known as “parallel lithography”.Parallel lithography generally prints an entire pattern at one time.This is usually accomplished by projecting photons through a mask onto aphotoresist-coated semiconductor wafer, as shown in FIG. 1. A mask(designated by an “M” in FIG. 1) provides a template of the desiredcircuit. A photoresist coat, which may be a thin layer of materialcoated on the wafer which changes its chemical properties when impingedupon by light, is used to translate or transfer the mask template ontothe semiconductor wafer. In more detail, mask M allows photons (incidentlight, designated by an “I”) to pass through the areas defining thefeatures but not through other areas. An example of a typical maskconstruction would be deposits of metal on a glass substrate. In a wayanalogous to the way light coming through a photographic negativeexposes photographic paper, light coming through the mask exposes thephotoresist. The exposed photoresist bearing the pattern selectively“resists” a further process (e.g., etching with acid, bombardment withvarious particles, deposition of a metallic or other layer, etc.) Thus,this lithography technique using photoresist can be used to effectivelytranslate the pattern defined by the mask into a structural pattern onthe semiconductor wafer. By repeating this technique several times onthe same wafer using different masks, it is possible to buildmulti-layered semiconductor structures (e.g., transistors) andassociated interconnecting electrical circuits.

[0007] Parallel lithography as described above has the advantage that itis possible to achieve a high throughput since the whole image is formedat once. This makes parallel lithography useful for mass production.However, parallel lithography has the disadvantage that a new mask isrequired each time one desires to change patterns. Because masks canhave very complex patterns, masks are quite costly and susceptible todamage.

[0008] For mass production, parallel lithography is usually done using amachine known as a “stepper.” As schematically depicted in FIG. 1, astepper consists of a light source (“I”), a place to hold a mask (“M”),an optical system (“lenses”, “L”) for projecting and demagnifing theimage of the mask onto a photoresist-coated wafer (“W”), and a stage(“S”) to move the wafer. The process of exposing a wafer using a stepperis summarized in FIG. 2A, and is depicted from a side view in FIGS.2B-2E. In each exposure, a stepper only exposes a small part of thewafer, generally the size of one chip. Since there are often manyseparate chips on each wafer, the wafer must be exposed many times. Thestepper exposes the first chip (FIG. 2B), then moves (“steps”) over(FIG. 2C) to expose the next chip (FIG. 2D) and repeats this process(FIG. 2E) until the entire wafer is exposed. This process is known as“step and repeat” and is the origin in of the name “stepper.”

[0009] A stepper must also be capable of precisely positioning the waferrelative to the mask. This precise positioning (overlay accuracy) isneeded because each lithography step must line up with the previouslayer of lithography. A stepper spends a significant portion of its timepositioning the stage and the rest exposing the photoresist. Despite thegreat precision necessary, steppers must be capable of high throughputto be useful for mass production. There are steppers that can processone hundred 8-inch wafers per hour.

[0010] One way to increase the usefulness of a chip is to increase itssize. In the “step and repeat” example described above, the size of thechip is limited to the exposure size of the stepper. The exposure sizeis small (roughly 20 mm×40 mm) because of the cost of an optical systemthat is capable of projecting a high quality image of the mask onto thewafer. It is very expensive to increase the size of a chip by increasingthe exposure size of the stepper (for example, this would require alarger lens-which by itself can cost many hundreds of thousands ofdollars). Another approach is to modify a stepper so that light onlyshines on a subsection of the mask at a given time. Then, the mask andwafer can be scanned (moved relative to the fixed light source)simultaneously until the entire mask is imaged onto the wafer, as inFIGS. 3A-3C. This modified stepper is known as a “scanner” or“scanner/stepper”.

[0011] Scanners offer increased chip size at the expense of increasedcomplexity and mask costs. Because scanner masks are larger, the masksare more fragile and are more likely to contain a defect. The increasedsize and fragility of the mask mean that the masks for a scanner will bemore expensive than the masks for a stepper. Also, because the image isbeing demagnified, the mask and wafer must be scanned at differentspeeds, as depicted by the length of the arrows in FIGS. 3A-3C. Becauseof the great precision required, differential scanning increases thecost and complexity of a scanner when compared with a stepper.

[0012] Many chip manufacturers are looking toward future improvements inresolution and/or exposure size to help continue the growth that hasdriven the semiconductor industry for the past thirty years. Theimprovements in these areas have been partly the result of improvementsin the optical systems used to demagnify the mask and of the use ofshorter wavelength light. In particular, modern lithography systems usedfor mass production are “diffraction limited”, meaning that the smallestfeature size that it is possible to print is determined by thediffraction of light and not by the size of features on the mask. Inorder to improve the resolution, one must use either a shorterwavelength of light or another technique such as optical proximitycorrection or phase shifting.

[0013] Another option for improving resolution is to put the mask incontact with the wafer, as in FIG. 4; the effects of diffraction can belessened by not giving the light a chance to “spread out” after itpasses through the mask. Unfortunately, contact lithography is notsuitable for mass production for at least two reasons. First, the maskmust now be the same size as the final pattern, making the mask moreexpensive and more fragile. Second, because the mask is in contact withthe wafer, it is easily damaged.

[0014] The present invention overcomes many of the disadvantages ofprior lithographic microfabrication processes while providing furtherimprovements that can significantly enhance the ability to make morecomplicated semiconductor chips at lower cost.

[0015] One aspect provided by this invention provides a new type ofprogrammable structure for exposing a wafer. The programmable structureallows the lithographic pattern to be changed under electronic control.This provides great flexibility, increasing the throughput anddecreasing the cost of chip manufacture and providing numerous otheradvantages.

[0016] The programmable structure provided in accordance with oneexample embodiment of the invention consists of an array of shuttersthat can be programed to either transmit light to the wafer (referred toas its “open” state) or not transmit light to the wafer (referred to asits “closed” state). A simplified example lithography systemincorporating such a programmable mask is schematically depicted in FIG.5A exposing an example pattern. In FIG. 5B the same programmable maskPPM is shown exposing a different pattern.

[0017] The programmable mask shown in FIGS. 5A and 5B can provide atwo-dimensional array of individual shutters each of which can beprogrammed to either transmit light (“open”, “transparent”) or blocklight (“closed”, “opaque”). At least one such two-dimensional array ofstructures can be placed between a wafer and a source of electromagneticenergy. Each of the structures may comprise an active region supportingan electron distribution that can be changed to affect the modulation ofelectromagnetic energy from said source. The structures can becontrolled to selectively modulate, in accordance with a programmablepattern, electromagnetic energy impinging on the wafer.

[0018] In accordance with this aspect provided by the invention, asystem for exposing a wafer may comprise a source of electromagneticenergy, a collimating lens optically coupled to the electromagneticenergy source, a wafer stage, and a two-dimensional array of structuresdisposed between the wafer stage and the collimating lens. Each of thestructures in the array may comprise an active region supporting anelectron distribution that can be changed to affect the modulation ofelectromagnetic energy from said source. An electrical controllercoupled to the two-dimensional array may be used to electrically controlthe semiconductor structures to selectively modulate, in accordance witha changeable pattern, electromagnetic energy from the source that isdirected toward the wafer stage.

[0019] In accordance with a further aspect provided by the presentinvention, the programmable structure can comprise or include an arrayof selective amplifiers. In accordance with this aspect provided by theinvention, a programmable electromagnetic energy modulating structurecomprises a two-dimensional array of solid-state selective amplifierseach comprising regions of permanently opaque material and activeregions. Control circuitry disposed within the array can be provided toselectively control each of the active regions to toggle between anamplifying state and a non-amplifying state. Thus, each selectiveamplifier is programmed to either amplify light (somewhat analogous tothe “open” or “transparent” state of a shutter) or be “non-amplifying”(its “closed” or “opaque” state). In the non-amplifying state, someportion of the incident light is transmitted through the amplifiermaterial. The portion of incident light that is transmitted through theamplifier can range from 0-100%, depending on the specific design andoperating conditions. Selective amplification has all of the advantagesof a programmable structure that uses shutters with several addedadvantages—including reduction in the time required to expose theresist.

[0020] In accordance with a further aspect provided by the invention theshutters and selective amplifiers can work in tandem to form a“programmable layer”. When the programmed pattern calls for light topass (or, not pass) through a particular pixel, both the selectiveamplifier and shutter corresponding to that pixel would be put intotheir open (or, closed) state. FIGS. 6A-6F schematically depicts theoperation of an example shutter (labeled as “SH”) (FIGS. 6A-6B), anexample selective amplifier (labeled as “AM”) (FIGS. 6C-6D), and anexample device (labeled as “X”) combining the two (FIGS. 6E-6F). In eachof these figures, “I” represents the intensity of light incident on theshutter/amplifier, and “I” represents the light intensity afterinteracting with the shutter/amplifier. Combining a selective amplifierwith a shutter in this manner achieves increased contrast over selectiveamplification alone.

[0021] In accordance with another aspect provided by the presentinvention, a programmable technique is provided for creating a patternto be imaged onto a wafer that can be implemented as a viable productiontechnique. Thus, the present invention also provides a technique ofmaking integrated circuits. In accordance with this aspect provided bythe invention, a wafer having a surface covered with photoresist isplaced on a movable wafer stage. A source directs electromagnetic energytoward a two-dimensional array of semiconductor structures disposedbetween the source and the wafer stage. The electron distribution withinthe structures is electrically controlled to define a desiredmicrofabrication exposure pattern that modulates electromagnetic energyfrom said source that impinges on the wafer in accordance with apattern. The modulated energy is used to expose the photoresist with thepattern. The wafer is then etched to selectively remove portions of thephotoresist based on the desired microfabrication exposure pattern, andthe etched wafer is treated to construct a semiconductor structure layeron the wafer.

[0022] In accordance with a further aspect provided by this, invention,a diffraction limiter can be used to provide certain advantagesassociated with contact lithography without requiring some of thedisadvantages of contact lithography. In accordance with this aspect ofthe invention, the diffraction limiter may provide an opaque layer inwhich there is an array of transparent regions (“holes”) distributed ina one-to-one correspondence to the selective amplifiers/shutters. Thediffraction limiter is placed in contact with the wafer, and the lightthat passes through the programmable layer is incident upon it. Thediffraction limiter allows the advantages of contact lithography whilemaintaining the distance between the programmable layer and wafer.

[0023] In accordance with a further aspect provided by the presentinvention, a programmable shutter array, a programmable selectiveamplifier array, and a diffraction limiter can all be used in a commonsystem. For example, FIG. 7A depicts schematically a lithography setupincorporating the above three components. FIG. 7B shows a zoomed-in viewof the diffraction limiter (denoted by “D”) and wafer section. Thesethree components provide a programmable lithography system that offershigh throughput, extremely accurate pattern reproduction, and excellentresolution.

[0024] Implementing a diffraction limiter in conjunction with anyprogrammable lithography system should significantly reduce thedisadvantages associated with contact lithography. This device is placedin contact with the wafer and thus reduces the effects of diffraction(see FIGS. 7A and 7B.) Even though the diffraction limiter is close tothe wafer and could be damaged, it is inexpensive and is easilyreplaced. Additionally, one can apply techniques such as phase shiftingand/or optical proximity correction on the diffraction limiter itself.Because of the simple, regular shape of the pixels on the diffractionlimiter, such corrections should be easily optimized.

[0025] Lithography in accordance with the present invention potentiallyallows a high throughput to be achieved. For example, only a singleprogrammable structure is necessary to print any desired pattern. Anon-exhaustive list of some of the many features and advantages providedby the present invention are as follows:

[0026] Programmable lithography offers increased flexibility overconventional parallel lithogaphy. This increased flexibility means thata greater variety of chips can be easily produced. It also opens up waysto improve the manufacture of all types of semiconductor products. Italso simplifies the entire process of designing and manufacturingsemiconductor products.

[0027] No need to have different masks to produce different chips.Because a single programmable structure can be programmed with anarbitrary pattern, it is no longer necessary to fabricate (and purchase)new mask sets in order to print new chips. This is extremelycost-effective for producing small quantities of specialized chipsbecause the cost of a mask set can be prohibitively expensive. Inproducing large quantities of chips the cost of the mask set is lesssignificant because it is a fixed cost amortized over many more chips.

[0028] Electronic alignment. Because it is extremely important to lineup the current lithography step with previous steps; steppers spend asignificant amount of time mechanically aligning the wafer with themask. With programmable lithography the pattern can be programmed intothe “programmable layer” (i.e. the programmable area within the mask)such that it is aligned with the wafer. This saves time over having tomechanically align as in the case of a conventional mask.

[0029] Disconnecting the size of the chip from the exposure size of thestepper. In conventional parallel lithography, the size of a chip isdetermined by the exposure size of the stepper. However, withprogrammable lithography a different pattern can be loaded into theprogrammable layer at each exposure. Hence, there is no longer anyreason that the same pattern needs to be imaged each time the stepperdoes an exposure. Consequently, in programmable lithography, the size ofthe chip is not intrinsically determined by the size of each individualexposure. This “disconnect” between chip and exposure size is asignificant advantage of programmable lithography because chipperformance and chip size are closely tied together.

[0030] Simplified optical proximity correction. Optical proximitycorrection is a technique that is used to increase the resolution of theoptical system at a given wavelength of light. Because of diffraction,the pattern on the mask is not faithfully reproduced on the wafer. Inorder to compensate for this, the pattern on the mask can be altered toaccount for diffraction so that the desired pattern can be imaged ontothe wafer. One problem with this technique in a conventional mask isthat it is difficult to decide how to alter the shape on the mask suchthat the desired shape will appear on the wafer. This is mainly due tothe size and complexity of the desired pattern. Programmable lithographygreatly simplifies this problem because in programmable lithography thesame shape (e.g., a square) is always being imaged onto the wafer byeach pixel, and the correction can be made on a pixel-by-pixel basis.

[0031] Simplified phase shifting. As with optical proximity correction,phase shifting is also a technique that is used to increase theresolution of the optical system at a given wavelength of light. In thistechnique, a material that causes a phase shift in light is placed onthe mask. The phase shifting material causes destructive interference atthe wafer between light from neighboring features in order to eliminatethe diffraction tails. Phase shifting also suffers from the same problemas optical proximity correction; due to the size and complexity of thepattern it is difficult to decide where to place the phase shiftingmaterial. As with optical proximity correction, programmable lithographyallows this problem to be greatly simplified because the same shape isalways being imaged onto the wafer at each pixel. Programmablelithography also allows the possibility of active phase shifting. Inactive phase shifting, each pixel would contain an additional layer inwhich there would be a material whose index of refraction changes when avoltage is applied.

[0032] Simplification of the chip making process. One of the bigproblems facing chip manufacturers is the growing complexity of the chipmaking process. With each new chip the manufacturer must get a brand newmask set. They must also inspect and repair these masks. Withprogrammable lithography only a single programmable structure is neededto produce an entire chip. In the event that a programmable mask breaksit can simply be replaced with another identical programmable mask.Additionally, programmable lithography will facilitate research anddevelopment of new products because of the greater ease of producingprototype devices.

[0033] The shutters and/or selective amplifiers can be fabricatedeasily. Each individual shutter can be a device similar to devices thatare typically used in chips themselves. This allows the design andfabrication of the shutter to draw on the enormous amount of knowledgeassociated with production techniques and operation of these devices.

[0034] The shutters and/or selective amplifiers can be small and denselypacked. Small shutters mean that the lithography system can producesmall features without the need for demagnification, although it couldbe used in a system that does include demagnification. Densely packedshutters mean high throughput because they reduce the number ofexposures necessary to expose the entire wafer.

[0035] The shutters and/or selective amplifiers can work for shortwavelength light. Shorter wavelengths provide better resolution, whichis important for chip performance.

[0036] The shutters and/or selective amplifiers generally will not breakduring normal operation. This is important because the mask must be ableto flawlessly reproduce the desired image. If even a single pixel isincorrect then the entire chip is likely to be worthless.

[0037] The shutters and/or selective amplifiers can switch statesquickly. The speed of the shutters is relevant for throughput and maybecome significant when many shutters must be addressed.

[0038] Selective amplifiers can be used alone and/or in combination withprogrammable shutters. An array of selective amplifiers can be used inthe place of or in addition to a PPM to project more light onto thewafer in some areas than in others corresponding to the pattern to beimaged. Or, an array of selective amplifiers can be used in astand-alone system, e.g., when the non-amplified light is not sufficientto expose the resist and the amplified light is.

[0039] Programmable lithography provides a resolution and throughputcomparable to conventional parallel lithography while retaining all ofthe advantages of programmable lithography.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] These and other features and advantages provided by the presentinvention will be better and more completely understood by referring tothe following detailed description of presently preferred exampleembodiments in conjunction with the drawings, of which:

[0041]FIG. 1 shows an example prior art technique of parallellithography using a stepper;

[0042]FIG. 2A summarizes the operation of an example prior art stepper;

[0043]FIGS. 2B to 2E show the simplified operation of an example priorart stepper;

[0044]FIGS. 3A to 3C show the simplified operation of an example priorart scanner;

[0045]FIG. 4 shows an example prior art contact lithography technique;

[0046]FIGS. 5A and 5B show a simplified example of technique oflithography in accordance with a preferred embodiment of the presentinvention using a programmable structure;

[0047]FIGS. 6A and 6B illustrate example operation of shutters providedin accordance with the present invention;

[0048]FIGS. 6C and 6D illustrate example operation of selectiveamplifiers provided in accordance with the present invention;

[0049]FIGS. 6E and 6F illustrate example operation of a combinationshutter/selective amplifier array in accordance with the presentinvention;

[0050]FIG. 7A shows an example lithography setup in accordance with apresently preferred example embodiment of the present invention;

[0051]FIG. 7B shows a zoomed-in view of the diffraction limiter andwafer of FIG. 7A;

[0052]FIG. 8 shows a more detailed example embodiment of the inventionand its components in accordance with the present invention;

[0053]FIG. 9 shows an example single selective amplifier and itscomponents in accordance with the present invention;

[0054]FIG. 10 shows an example single shutter and its components inaccordance with the present invention;

[0055]FIG. 11 shows an example single programmable pixel and itscomponents in accordance with the present invention;

[0056]FIGS. 12A to 12H show an example technique of programmablelithography for a “4-step programmable layer” in accordance with thepresent invention;

[0057]FIG. 13A summarizes an example electronic alignment technique inaccordance with the present invention;

[0058]FIGS. 13B to 13G shows an example electronic alignment techniquein accordance with the present invention; and

[0059]FIGS. 14A to 14F show an example technique in accordance with thepresent invention for printing chips that are larger than the exposuresize of the mask.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0060] In an example preferred embodiment, the invention consists ofthree main components: an array of selective amplifiers 10, an array ofsolid-state shutters 12, and a diffraction limiter 14. Taken together,the array of amplifiers 10 and the array of shutters 12 form what weshall call a “programmable layer”, PL. FIG. 8 shows these threecomponents incorporated into an example setup for performinglithography. In this figure, incident light, I, shines on theprogrammable layer from above. A pattern generator feeds theuser-defined pattern into the programmable layer.

[0061] The array of selective amplifiers 10 consists of regions of apermanently opaque material 22 (a metal such as aluminum) and activeregions 20. Each active region 20 within the array of selectiveamplifiers 10 is associated with a single selective amplifier 40. Theactive regions 20 can be toggled between a non-amplifying 20 a andamplifying 20 b state via control circuitry (designated by an encircledC1).

[0062] Following the array of selective amplifiers 10 is an array ofsolid-state shutters 12. The array of solid-state shutters 12 consistsof regions of a permanently opaque material 22 and active regions 24.Each active region 24 within the array of shutters 12 is associated witha single shutter 60. The active regions 24 can be toggled between anopaque 24 a and a transparent 24 b state via control circuitry(designated by an encircled C2).

[0063] For reference, a given selective amplifier 40 and itscorresponding solid-state shutter 60 will be taken as a “programmablepixel” 100.

[0064] A diffraction limiter 14 is placed in contact with aresist-coated wafer. The diffraction limiter 14 is comprised of regionsof a transparent material 30 (such as single crystal sapphire) andregions of an opaque material 32 (a metal such as aluminum). In thepreferred embodiment, this is accomplished by depositing opaque materialsuch as aluminum onto a transparent substrate such as sapphire. Thetransparent regions individually correspond to the locations of theactive regions of the programmable pixels in the programmable layer.

[0065]FIG. 9 details a single selective amplifier 40. Each singleselective amplifier 40 consists of a section (46, 48, 50, 52) capable ofselectively amplifying a beam of light, the circuitry C1 needed tocontrol the state of the selective amplifier, and a material 22 thatblocks the light between adjacent selective amplifiers. In the preferredembodiment, the section capable of selectively amplifying light is ajunction 52 between a p-type semiconductor 48 (such as p-doped GaN) andan n-type semiconductor 50 (such as n-doped GaN) with metal contacts 46(such as aluminum) that are used to bias the junction 52. The controlcircuitry C1 would be capable of turning the bias on or off. The planeof the p-n junction 52 is oriented such that the normal of the plane isperpendicular to the incoming light. The p-n junction 52 is exposed tothe incident light through a hole 44 in the permanently opaque region22. The semiconductor materials 48 and 50 must be chosen such thatstimulated emission occurs for the wavelength of incident light beingused.

[0066]FIG. 10 details a single solid-state shutter 60. Each singlesolid-state shutter 60 consists of a section (66, 68, 70, 72) capable ofeither blocking or transmitting the incident light, the circuitry C2needed to control the state of the shutter, and a material 22 thatblocks the light between adjacent shutters. In the preferred embodiment,the section capable of either blocking or transmitting light is a MOSstructure: an insulator region 68 (such as SiO₂) is sandwiched betweenmetal electrodes 66 (such as aluminum) and a semiconductor region 70(such as n-doped (n⁺⁺) GaN). Control circuitry C2 is used to bias theMOS structure across the electrodes 66. The active region 72 is exposedto the incident light through a hole 64 in the permanently opaque region22.

[0067]FIG. 11 details a programmable pixel 100. Each programmable pixel100 consists of a single selective amplifier 40, a single shutter 60, atransparent insulator 102 (such as SiO₂) between the amplifier andshutter, the control circuitry, represented by an encircled C1 and C2,needed to control the state of the pixel, and a material 22 that blocksthe light between adjacent pixels. The active region of the amplifierportion 52 must overlap the active region of the shutter 72 to form theactive region of the pixel 104. The active region of the pixel 104 isexposed to the incident light through a hole 106 in the permanentlyopaque region 22.

[0068] The combined control circuitry, C1 and C2, needs to be capable oftoggling a voltage across each individual pixel 100 depending on thepattern supplied by the user via the pattern generator. This controlcircuitry is similar to the standard type of addressing used in memorydevices or an LCD.

[0069] The above-described example embodiment is specified as to beuseful in a lithography system utilizing (approximately) 385 nm (andlonger) wavelength light. However, there are many other possibleembodiments. For instance, the choice of specific materials for thevarious semiconductors and insulators can render the system useful forother wavelengths of incident light. Specifically, the band gap of thesemiconductor in the selective amplifier determines the wavelengths oflight that can be amplified. Of course, the materials chosen for theamplifier must be capable of amplifying light. For example, this couldbe accomplished in the manner described in the ensuing theory ofoperation section. Likewise, for the shutter, the materials used can beoptimized for operation with a selected wavelength of incident light,again described in the theory of operation section.

[0070] Also, the specific geometry of electrodes and active materialscould be modified while still retaining the usefulness of the invention.For instance, all the electrodes could be placed on the top surface ofthe device. Or, for example, the orientation of the plane of thejunctions does not necessarily need to be completely perpendicular tothe incoming light. This could simplify the fabrication process of thepixels.

[0071] Furthermore, since shorter wavelengths are desirable inlithography, materials with a large band gap are desirable for use inthe pixels. In fact, some materials that may not typically be thought ofas semiconductors may be useful for our purposes because they have alarge band gap, such as sapphire, diamond, SiO₂, LiF, ZnS, AIN, ZnSe,etc. . . Additionally, in the amplifier section, our invention useselectric fields to induce a population inversion, but this is not theonly possible means of doing so. Other techniques include but are notlimited to optical pumping, thermal pumping, etc. . . Similarly, in theshutter section, our invention uses electric fields to change thedensity of occupied states, but this is not the only possible means fordoing so. Other techniques include but are not limited to changing thetemperature of the semiconductor or shining light onto thesemiconductor.

[0072] There is a large field of research devoted to controllingpopulation densities for the purpose of creating, manipulating,blocking, or amplifying light. One active area of interest in this fieldis the creation of semiconductor lasers. Many of the structures createdfor semiconductor lasers or optical amplifiers could be used forselective amplification (in fact, semiconductor lasers could be used inthe place of the selective amplifiers, in which case an external lightsource would not be necessary.) These other structures would include butwould not be limited to heterostructures and vertical cavity surfaceemitting lasers (VCSELs). Similar structures could also be used for theshutter. While these other structures operate on similar principles tothose described in the theory of operation section, they might providesome advantages, like decreasing the amount of voltage or currentrequired to operate the device, or increasing the area of the activeregion.

[0073] There are also many possible alternate embodiments for thediffraction limiter. The materials must merely satisfy the requirementthat the transparent regions individually correspond to the locations ofthe active regions of the programmable pixels in the programmable layerwith opaque regions in between. For instance, the transparent regionscould be physical holes etched through an opaque substrate, or, thetransparent and opaque regions could be semiconductors with varyinglevels and/or kinds of doping.

[0074] Example Operation of Preferred Embodiments

[0075] In accordance with the preferred embodiments described above,each individual pixel 100 has the ability to either block or amplifylight, I, and is able to be toggled between the two settings by means ofthe control circuitry C1 and C2. The pattern generator will control theindividual pixels 100 in such a way that the control circuitry C1 and C2always act in tandem within a given pixel.

[0076] The pattern generator addresses the control circuitry C1 tocontrol a potential difference across the metal contacts 46. If thesemiconductor 48 is doped n-type and the semiconductor 50 is dopedp-type, then a depletion region is created in the active region 52. Whena potential difference is applied across the contacts 46, a populationinversion is formed in the depletion(active) region 52. Incident lightof the appropriate wavelength causes stimulated emission and is thusamplified in the active region 52. If no potential difference is appliedacross the contacts 46, the light passes through the region 52unaltered. This satisfies our requirement for a selective amplifier.

[0077] The pattern generator simultaneously addresses the controlcircuitry C2 to control a potential difference across the metal contacts66. When a potential difference is applied, this changes both thedensity of states and the conductivity of the semiconductor 70 in theregion 72 adjacent to the semiconductor-insulator (70-68) interface. Forinstance, if the semiconductor 70 is doped n-type and a proper potentialdifference is applied across the metal contacts 66, then a depletionregion is created in the active region 72. In the depletion (active)region 72 the conduction band electrons are repelled by the electricfield created by the bias. The conductivity is dramatically decreased inthe depletion (active) region 72. The density of occupied states in thedepletion (active) region 72 is also changed because the states at thebottom of the conduction band that were previously filled are no longerfilled. Hence, incident light with energy less than the semiconductor 70band gap is transmitted through the device in the presence of apotential difference, and is absorbed in the absence of a potentialdifference. (Conversely, as per the theory of operation section, certainwavelengths of light with an energy greater (including x-rays) than thesemiconductor 70 band gap are transmitted through the device in theabsence of a potential difference, and are absorbed in the presence of apotential difference.) This satisfies our requirement for a shutter.

[0078] In order to utilize the programmable layer, PL, one must do aseries of exposures of the photoresist 33. Between each exposure twoactions can be taken: (a) any or all of the pixels 100 can be toggled toa new state, and/or (b) the substrate can be moved, if necessary. Byselecting which pixels 100 to make opaque or transparent themanufacturer can generate any pattern that he or she desires. FIGS. 12Ato 12H, depict an example of this process using what we shall call a“4-step programmable layer.” The term “4-step” means that in order tocreate a pattern that fills the entire area of the mask, the substratemust be exposed four times. Each step is shown in a different pair offigures, one representing a perspective view and the other representinga top view. For example, FIGS. 12A and 12B both depict the firstexposure step, from differing views. In the example depicted, theprinting of the desired pattern is completed in the last step, shown inFIGS. 12G and 12H. Another case would be if the programmable layer weredesigned such that the desired pattern could be printed without needingto move the substrate at all (similar to the setup in FIG. 7A).Lithography using this “1-step programmable layer” would act as a fullyparallel process.

[0079] A light source, I, illuminates the programmable layer, PL, fromabove and the light transmitted 31 by the layer passes through a lens,L, and is demagnified. After demagnification, the image of theprogrammable layer, PL, impinges on the diffraction limiter 14 which isheld in close proximity with a substrate coated with a photosensitive toresist 33 as in FIG. 8. Light 31 is only transmitted through thediffraction limiter in transparent regions 30 and not through regionscovered by opaque material 32, and thus exposes the resist in regions35. The same setup is depicted in a more schematic way in FIG. 7A forpurposes of clarity.

[0080] One example technique for electronic alignment is summarized inFIG. 13A. Fig 13B shows a wafer, W, that has been previously processed.FIG. 13C shows the pattern that the user intends to add and its positionrelative to the previously processed features. FIG. 13D shows theprogrammable layer, PL, coarsely aligned with the wafer. At this stage,the programmable layer is not yet aligned to with a pixel, and if anexposure were made the resulting pattern would be misaligned with thepreviously processed features as in FIG. 13E. FIG. 13F shows theprogrammable layer after alignment to within a pixel, and the subsequentsuccessful exposure. Given a different coarse alignment, FIG. 13G showsthe case in which a different individual pixel in the programmable layerhas been aligned with the previously processed features, and thesubsequent successful exposure. FIGS. 13F-13G together demonstrate thatthe absolute location of the programmable layer relative to the wafer isnot important as long as the programmable layer is aligned to within anysingle pixel. The pattern can be electronically shifted to a new set ofpixels in the programmable layer such that they will project the imagein the correct location.

[0081] An example of how to use an example preferred embodiment providedin accordance with the present invention to disconnect the chip sizefrom the exposure size is summarized in FIG. 14A. FIG. 14B shows thepattern that the user intends to print on the wafer, W. Note that thispattern is about four times the single exposure size of the programmablelayer, PL. In each of the FIGS. 14C-14F, the programmable layer isloaded with the proper section of the total pattern, an exposure ismade, and the wafer is moved to the left by a fixed amount. In FIG. 14F,the desired pattern has been printed.

[0082] Theory of Operation

[0083] We begin with a discussion of an example shutter in accordancewith the present invention, followed by a discussion of an exampleselective amplifier in accordance with the present invention.

[0084] For the preferred embodiment array of shutters to work, it isnecessary that the various shutters can be made either “transparent” or“opaque” to the incident light. For a mask, both terms “transparent”.and “opaque” describe the ratio of the intensity of incident light tothe intensity of transmitted light. An “opaque” material is differentrelative to a “transparent” material in that the amount of lightattenuated is much greater in the case of the “opaque” material. Theamount of attenuation is best measured using the concept of anattenuation coefficient, α, defined by the relation $\begin{matrix}{{\frac{I}{I_{o}} = ^{{- \alpha}\quad z}},} & (1.)\end{matrix}$

[0085] where I is the transmitted intensity, I_(o) is the incidentintensity, and z is the thickness of material traversed. Using this, ifwe compare the transmitted intensities through a “transparent” materialand an “opaque” material, we get a measure of the contrast in lightimpinging on the resist, C, as $\begin{matrix}{C = {\frac{I_{transparent}}{I_{opaque}} = {\frac{^{{- \alpha_{transparent}} \cdot z}}{^{{- \alpha_{opaque}} \cdot z}} = {^{{+ {({\alpha_{opaque} - \alpha_{transparent}})}} \cdot z}.}}}} & (2.)\end{matrix}$

[0086] If this contrast is a large number, the resist will only beactively exposed when the shutter is in the “transparent” state. Thus,if we can control the attenuation coefficient in a material, i.e. varyit from α_(transparent) to α_(opaque) so that their difference isappreciable, then the system is a useful candidate for a programmablemask.

[0087] The attenuation coefficient, α, arises from all possiblemechanisms of absorption of the light combined. For our system, the twomechanisms we are most interested in are (a) absorption by “free”carriers in the conduction band, and (b) atomic/interband photoeffect.

[0088] Absorption by “Free” Carriers in the Conduction Band

[0089] In a simple classical model of conduction, the conductivity of amaterial is directly proportional to the number of free electrons perunit volume available to interact. These charges are capable ofabsorbing or reflecting the incident light. If the fields of the lightare harmonic as e^(i({overscore (k)}·{overscore (x)}−ωt)) then we canwrite the wavevector as $\begin{matrix}{{k = {\beta + {i\frac{\alpha}{2}}}},{where}} & (3.) \\{{ \begin{matrix}\beta \\\frac{\alpha}{2}\end{matrix} \} = {\sqrt{\mu \quad ɛ}{\frac{\omega}{c}\lbrack \frac{\sqrt{1 + ( \frac{4\pi \quad \sigma}{\omega ɛ} )^{2}} \pm 1}{2} \rbrack}^{1/2}}},} & (4.)\end{matrix}$

[0090] where μ is the magnetic permeability, ∈ is the dielectricconstant, σ is the conductivity of the material, c is the speed oflight, and ω=E/h is the circular frequency of the light.

[0091] Inserting Eq. (4.) into Eq. (3.), and using thee^(i({overscore (k)}·{overscore (x)}−ωt)) dependency, we see that theamplitude of the light incident in the z direction will drop offexponentially as e^(−αz/2) and hence the intensity will go as e^(−αz),verifying that α is indeed the attenuation coefficient mentionedearlier. Hence, a change in the conductivity, σ, in Eq. (4.) will resultin a corresponding change in the attenuation coefficient, α.

[0092] Although the above classical treatment is not sufficient todescribe the details of the interactions of photons with the conductionband electrons, the qualitative property of the conductivity affectingthe absorption coefficient still holds true. For a more completedescription, one must treat the system quantum mechanically. The nextsection deals with the quantum mechanical interaction of light with asolid.

[0093] Absorption Via Atomic/interband Photoeffect

[0094] In this case, an electron absorbs an incident photon and ispromoted to an excited state. If the energy of the light is insufficientto liberate it from the material completely (photoelectric knockout),then the electron must be promoted into another quantized state in thematerial. If such a state does not exist, the incident photon is notabsorbed because energy cannot be conserved. Furthermore, if the statedoes exist but is already occupied by another electron (ignoring spin),the transition is also forbidden by the Pauli principle. If thetransition is allowed, the probability that such an interband transitionwill occur can be written

P _(if)(E _(y)) ∝T _(if) g _(f)(E_(f)) g _(i) (E_(i)), with E _(f) =E_(i) +E _(y)  , (5.)

[0095] where T_(if) is the transition matrix element, g_(i) (E_(i)) isthe density of states initially occupied by electrons, and g_(f) (E_(f))is the density of unoccupied final states into which the electron may bepromoted. The attenuation coefficient is proportional to thisprobability, P_(if). Hence, if one can change either g_(i) or g_(f) thenone can effectively make the material “transparent” or “opaque”.

[0096] To illustrate how the absorption can be controlled in a shutterwe now look to specific examples.

[0097] We will consider a Metal-Oxide-Semiconductor device (“MOS”) as anexample, but the discussion could also be applied to other structuressuch as p-n junctions, and even insulators. The region of interest willbe in the semiconductor, near the interface with the oxide. Thesemiconductor has a band gap which we will choose to be 1 eV. In theintrinsic case, light with an energy less than 1 eV incident on thesemiconductor is not able to promote electrons from the valence band tothe conduction band. Hence, interband transitions will not contribute tothe attenuation coefficient. The “free” electrons in the conduction bandsolely interact with the incident light. With this, if we consider thecase where we dope the semiconductor as n-type (i.e. adding more freeelectrons to the conduction band), more light will be absorbed as weincrease the dopant concentration (because there are more electronsinteracting). For light energies less than the band gap, we willconsider the material to then be “opaque” to the light, as the electronsin the conduction band absorb a significant amount of the incidentintensity.

[0098] Next, if we apply a voltage to the MOS structure, we create aregion near the interface in which the number of conduction bandelectrons is reduced compared to the non-biased case. Now we have asituation where the amount of light absorbed is less, because we have ineffect changed the conductivity of the semiconductor in this depletionregion. This situation is “transparent” relative to the aforementioned“opaque” setup. The actual materials used and the technique for creatingthe depletion region should be chosen in such a way to maximize thecontrast, C, while keeping the actual transmitted intensity,I_(transparent), as large as possible (so that the process takes aslittle time as possible.)

[0099] Another example would be to choose incident light with an energyjust larger than the band gap energy. Then, interband transitions arelikely to dominate the absorption. In the same n-type doped MOSstructure as above, the density of unoccupied states in the conductionband, g_(f), into which a valence band electron can be promoted isreduced near the bottom of the conduction band. This is due to thepresence of a large concentration of electrons from the dopant atoms.Now, consider the situation in which we apply a voltage across theinterface. This creates a depletion region in which there are fewerelectrons present in the conduction band, and hence more unoccupiedstates. Comparing the biased and unbiased situations, one can see thatthe latter should be more “opaque” than the former, because more valenceband electrons can be promoted into the newly vacated conduction bandstates. Once again, materials and voltages, etc. . . can be chosen tomaximize both contrast and “transparent” intensity. Also, if we had usedp-type doping rather than n-type, a corresponding change in g_(i) wouldhave appeared. Depleting the region of holes would have the same effectof toggling the state of the shutter. One should note that in thepreviously stated examples, the attenuation coefficient is onlyappreciably changed for a specific range of light energies, determinedby properties such as but not limited to the choice of materials in theMOS structure, the bias supplied, and the dopant density. It should alsobe noted that this transition-blocking effect can work for photonenergies corresponding to any transition within the material. Forexample, changing the density of unoccupied states in the conductionband, g_(f), can affect the absorption of light (x-rays) on inner shellelectrons in a material. Higher energies of light are desirable inlithography because the effects of diffraction are reduced as the lightenergy is increased.

[0100] Next, for the preferred embodiment array of selective amplifierswork, it is necessary that the various selective amplifiers can be lomade either “amplifying” or “non-amplifying” to the incident light. Fora mask, both terms “amplifying” and “non-amplifying” describe the ratioof the intensity of incident light to the intensity of output light. An“amplifying” state is defined relative to a “non-amplifying” state inthat the amount of light at the output is much greater. Using this if wecompare the output intensities through an “amplifying” material and a“non-amplifying” material, we get a measure of the contrast in lightimpinging on the resist, C, as $\begin{matrix}{{C = {\frac{I_{amplifying}}{I_{{non} - {amplifying}}} = \frac{I_{amplifying}}{I_{o}}}},} & (6.)\end{matrix}$

[0101] where I_(non-amplifying) is the incident intensity I_(o), andI_(amplifying) is the output intensity. The contrast is typically anincreasing function of the thickness of the selective amplifier.

[0102] If this contrast is a large number, the resist will only beactively exposed when the selective amplifier is in the “amplifying”state. Thus, if we can control the amount of amplification in a materialsuch that C is large, then the system is a useful candidate for use in aprogrammable mask.

[0103] The amplification occurs by a process known as stimulatedemission. If an electron exists in a state that is an energy ΔE aboveanother state, then when the electron drops (makes a transition) fromthe higher state to the lower state it will emit a photon with energyΔE. A photon with energy ΔE that passes near the electron in the excitedstate can cause the electron to drop to the lower state and emit aphoton with the same energy, phase, and direction as the first photon.This is the process of stimulated emission. Stimulated emission is awell-known process and is the basis of the laser. In order forstimulated emission to occur, several conditions must be met. Onecondition is that there must be a population inversion. Ordinarily,electrons are in the lowest state available to them. When there are moreelectrons in an excited state than in the lower state then a populationinversion exists. Another condition for stimulated emission is thatthere must be initial photons of the proper energy to cause theelectrons to drop from the excited state.

[0104] In the preferred embodiment of the array of selective amplifiers,biasing the p-n junctions causes population inversions and the incidentlight provides the initial photons necessary for stimulated emission.The light that is shined on the biased p-n junctions is thereforeamplified; the biased situation is the aforementioned “amplifying”state. The light that is incident on the unbiased p-n junctions istransmitted but not amplified. This allows the incident light to beselectively amplified by controlling which p-n junctions are biased. Aslong as there is appreciable amplification, and hence appreciablecontrast, selective amplifiers can be useful in a lithography system.

[0105] Although the preferred embodiment describes a complex device thatcombines various components, each component in itself represents eithera new technology or a great improvement upon existing technology. Forexample, an array of selective amplifiers or an array of shutters couldbe used as a stand-alone programmable structure, either with or withouta diffraction limiter. Additionally, either type of array or adiffraction limiter could be implemented as part of any programmablelithography scheme.

[0106] Therefore, while the invention has been described in connectionwith what is presently considered to be the most practical and preferredembodiment, it is to be understood that the invention is not to belimited to the disclosed embodiment, but on the contrary, is intended tocover various modifications and equivalent arrangements included withinthe spirit and scope of the appended claims.

We claim:
 1. An exposure system that exposes a substrate withelectromagnetic energy from an electromagnetic energy source, saidsystem comprising: a programmable mask comprising at least onetwo-dimensional array of structures that, in use, is disposed betweenthe substrate and the electromagnetic energy source, at least some ofsaid structures within said array having an active region comprising amaterial with a bandgap, the material being changed from transparent toopaque by application of a stimulus; and a controller coupled to themask, the controller controlling the stimulus applied to said structuresto cause the structures to interact with and selectively modulate, inaccordance with a programmable two-dimensional pattern, electromagneticenergy from the source so as to provide a two-dimensional programmableexposure pattern of electromagnetic energy exposing at least part of thesubstrate.
 2. The system of 1 wherein the material with a band gap ischosen from the group consisting of GaN, AIN, sapphire, diamond, SiO2,LiF, ZnS, and ZnSe.
 3. The system of claim 1 wherein the material haselectrons and holes, and the controller-applied stimulus alters thelight attenuation of the structures by changing the density of occupiedinitial states or the density of unoccupied final states of either theelectrons or the holes.
 4. The system of claim 1 wherein the controllerchanges the light attenuation of the structures by changing theconductivity thereof.
 5. The system of claim 1 wherein the substratecomprises a semiconductor wafer.
 6. A system for exposing asemiconductor wafer with electromagnetic energy from a source,comprising: a programmable photolithographic mask comprising at leastone two-dimensional array of structures disposed between the wafer andthe source of electromagnetic energy, at least some of said structureswithin said array comprising an active region comprising a materialwhich can be made transparent or opaque by applying a voltage to changethe density of occupied initial states or the density of unoccupiedfinal states of either the electrons or the holes; and a controllercoupled to the mask, the controller controlling a voltage applied tosaid structures to thereby cause the structures to interact with andselectively modulate, in accordance with a programmable two-dimensionalpattern, electromagnetic energy from the source so as to provide atwo-dimensional programmable exposure pattern of electromagnetic energyfor exposing at least part of the wafer.
 7. The system of claim 5wherein the material is chosen from the group comprising GaN, AIN,sapphire, diamond, SiO2, LiF, ZnS, and ZnSe.
 8. An exposure systemcomprising: a source of electromagnetic energy; a moveable stage thatsupports a substrate; optics that direct said electromagnetic energyfrom the source toward the stage a programmable photolithographic maskoptically coupled in an optical path between the source and the stage,the mask comprising at least one two-dimensional array of structures, atleast some of said structures within said array comprising an activeregion comprising a material which can be made transparent or opaque byapplying a voltage or current to change the density of occupied initialstates or the density of unoccupied final states of the electrons; andan electrical controller coupled to at least some of said structures,said controller applying a controlled voltage or current to saidstructures thereby causing said structures to interact with andselectively modulate, in accordance with a programmable two-dimensionalpattern, electromagnetic energy from the source so as to provide atwo-dimensional programmable exposure pattern of electromagnetic energyfor exposing at least part of a substrate disposed on the stage.
 9. Thesystem of claim 8 wherein the material is chosen from the groupcomprising GaN, AIN, sapphire, diamond, SiO2, LiF, ZnS, and ZnSe. 10.The system of claim 8 wherein said substrate comprises a semiconductorwafer.
 11. A wafer exposure system for exposing a wafer, said systemcomprising: a source of electromagnetic energy; a movable wafer stagethat supports and positions said wafer; a programmable photolithographicmask disposed between the source and the stage, the mask comprising atleast one two-dimensional array of structures, at least some of saidstructures within said array comprising a semiconductor active regionhaving holes comprising a material which can be made transparent oropaque by applying a voltage or current to change the density ofoccupied initial states or the density of unoccupied final states of theholes; and a voltage or current source that applies at least onecontrolled voltage or current to at least some of said structures; andan electrical controller coupled to the voltage or current source, thecontroller controlling the voltage or current said voltage or currentsource applies to said structures to thereby cause the structures tointeract with and selectively modulate, in accordance with aprogrammable two-dimensional pattern, electromagnetic energy from theelectromagnetic energy source to provide a two-dimensional programmableexposure pattern of electromagnetic energy exposing at least part of thewafer on the stage.
 12. The system of claim 11 wherein the material ischosen from the group comprising GaN, AIN, sapphire, diamond, SiO2, LiF,ZnS, and ZnSe.
 13. The wafer exposure system of claim 11 wherein thecontroller includes a digital processor.
 14. A method of exposing asubstrate comprising: placing a programmable photolithographic maskcomprising at least one two-dimensional array of structures between thesubstrate and a source of electromagnetic energy, at least some of saidstructures within said array including an active region comprising amaterial whose light attenuation coefficient is changed by applying avoltage or current to change the density of occupied initial states orthe density of unoccupied final states; and controlling a voltage orcurrent applied to said structures to interact with and selectivelymodulate, in accordance with a programmable two-dimensional pattern,electromagnetic energy from the source to provide a two-dimensionalprogrammable exposure pattern of electromagnetic energy exposing atleast part of the substrate.
 15. The method of claim 14 wherein thematerial is chosen from the group comprising GaN, AIN, sapphire,diamond, SiO2, LiF, ZnS, and ZnSe.
 16. The method of claim 14 whereinthe substrate comprises a wafer.
 17. The method of claim 14 wherein thesaid initial and final states are either electrons or holes.
 18. Amethod of exposing a substrate comprising: placing a programmablephotolithographic mask comprising at least one two-dimensional array ofstructures between a wafer and a source of electromagnetic energy, atleast some of said structures within said array comprising an activeregion comprising a material whose light attenuation coefficient can bechanged from transparent to opaque by applying a voltage or current; andapplying a controlled voltage or current to said structures to causesaid structures to interact with and selectively modulate, in accordancewith a programmable two-dimensional pattern, electromagnetic energy fromthe source so as to provide a two-dimensional programmable exposurepattern of electromagnetic energy for exposing at least part of thesubstrate.
 19. The method of claim 18 wherein the material is chosenfrom the group comprising GaN, AIN, sapphire, diamond, SiO2, LiF, ZnS,and ZnSe.
 20. The method of claim 18 wherein the substrate comprises asemiconductor wafer.
 21. The method of claim 18 wherein said appliedvoltage or current changes the conductivity of said active region.